The mode is the most common number in the data set. If there are two numbers that appear frequently (in this case 69 and 73) then this data set has two modes.
The median is the middle of the set of numbers (line them up from top to bottom).
In this case it should look something like:
68 68 69 69 69 69 70 70 71 72 72 73 73 73 73 75 78
If the whole list of numbers are even, then pick the two middle numbers and find their average. If it is odd, just choose the middle most number.
Mean is the average of all numbers. If you have 5 numbers, add them all up and divide by 5. If you have 10 numbers, add them all up and divide by 10.